1. Field of the Invention
This invention relates to a method and apparatus for measuring the density of bones. More specifically, it relates to a method and apparatus for measuring the density of bones that are inside a body, using an image sensor that comprises an active pixel sensor array.
2. Description of the Related Art
Osteoporosis is a disease in which the calcium content of a person's bones is gradually reduced. This leads to an increased risk of fractures, particularly common in post-menopausal women. It has been estimated that approximately 40,000 American women die per year from complications due to osteoporosis.
The mineral loss from a person's bones can be estimated from a single x-ray image of a body part. A more refined existing technique for detecting the mineral loss from a person's bones is dual-energy x-ray absorptometry (DEXA). DEXA uses two x-ray images obtained using x-rays of different energy levels to compensate for the fact that bones (hard tissue) are surrounded by skin, muscle, ligaments, etc. (soft tissue) that also contribute to the x-ray image.
In a single energy systems, it is impossible to determine which portion of the overall x-ray absorption was absorbed by the soft tissue, because any point in the image may lie underneath a combination of both hard tissue and soft tissue. In contrast to single energy systems, DEXA systems use two images to obtain a set of two simultaneous equations for each pixel in the images and then solve those equations to determine the amount of x-rays that was absorbed by the bone.
For example, U.S. Pat. Nos. 5,150,394 and 5,465,284, issued to Karellas, measures bone density using x-ray radiation at two different levels of energy. In Karellas patents, x-ray radiation of two intensity levels is transmitted through a portion of the patients body to a scintillator which converts the x-rays into visible light. The visible light emitted by the scintillator is provided to a charge-coupled device (CCD), which in turn converts the visible light into an electrical signal. The Karellas system forms an image of the body from the electrical signal, and determines the density the patient's bone from the image.
The primary problem of Karellas system stems from the use of a charge-coupled device (CCD) as the image conversion device. In a CCD, packets of electrical charges are stored in one of an array of discrete locations (known as "pixels"), with the amount of charge created and stored in each pixel corresponding to the intensity of light hitting the device at that location. The amount of charge stored in each pixel is read out by the successive application of control voltages to the device, which control voltages cause the packets of charge to be moved from pixel to pixel to a single output circuit. Through this process, the output circuit produces an analog electrical signal the amplitude of which at a given point in time represents the intensity of light incident on the device at a particular correspondence spatial location.
A CCD relies in its operation on the transfer of electrons from one pixel to another, a process that is often analogized to a "bucket brigade." Accordingly, before reaching the output circuit, the transferred electrons must pass though silicon for macroscopic distances, on the order of centimeters. Because of this, the ratio of electrons successfully transferred to the number left behind per electrode, the so-called "charge transfer efficiency" (CTE), must be as close as possible to perfect (i.e., no electrons left behind) to ensure acceptable performance of the CCD.
In addition, since net CTE varies exponentially with the number of charge transfers, the requirement for transfer efficiency becomes more stringent as CCD array sizes become larger. Also, manufacturing yield may decrease as the array size increases, since CCDs are vulnerable to single point defects that can block an entire column, rendering the entire device unusable.
CCDs also require special manufacturing techniques to achieve the required high CTE. As a result of the necessity of using such techniques, CCDs are not integratable with low power CMOS circuits, the technology most appropriate for low power integration of on-chip timing and driver electronics that is required for instrument miniaturization. Moreover, since CCDs require 12-26 volts of power, devices using this technology can present something of a shock hazard.
Recently, Active Pixel Sensor (APS) technology has provided an alternative to CCDs and other sensing devices for converting light into electrical signals. This technology is shown, for example, in U.S. Pat. No. 5,471,515 to Fossum et al., and hereby incorporated by reference. In general terms, an APS array is defined as an array of light sensors having one or more active transistors associated with each pixel. The transistors, which are the pixel's "active" elements, perform gain or buffering functions.
Because each pixel has its own active element, the charges that collect below each photosite need not be transferred through a "bucket brigade" during the readout period, as in a CCD. Thus, the need for nearly perfect charge transfer is eliminated. Accordingly, an APS array does not exhibit the negative attributes associated with charge transfer across macroscopic distances required by the CCD.
Also, since APS devices can be manufactured using standard CMOS techniques, the array can operate on 5 volt power, minimizing the shock hazards of the device. An additional advantage of utilizing APS technology in x-ray applications is that CMOS wafers are made in much larger diameter than are CCD wafers which allows large sensors to be manufactured more readily.
While APS arrays have of late enjoyed a good deal of attention from those constructing light detecting devices--such as in the high definition television (HDTV) and electronic still camera fields--they have not heretofore been used to construct an x-ray detector, such as a bone density measuring device. The reasons for this are several. To begin with, an x-ray detector is generally constructed by disposing a scintillator on top of a light sensing device, so that the scintillator first converts incident x-rays into visible light, and the light sensing device in turn converts the visible light into electrical signals. Some fraction of the x-rays that enter the scintillator, however, will invariably exit the scintillator and impinge upon the light sensing device. Such unconverted x-rays would be registered by conventional APS devices, and cause spurious signals to be created, which would, in turn, result in a noisy image.
In addition, the visible light emitted by scintillators is typically in the blue-green portion of the visible spectrum. APS arrays, however, are widely believed to exhibit a very poor response to blue-green light, leading in turn to the belief that APS arrays are not suitable for use in x-ray detectors.
Also, APS devices have a higher dark signal (i.e., thermally generated currents produced by the device when not exposed to radiation) than CCDs, since the dark signal in CCDs can be significantly reduced by operating the device in the multi-phase pinned (MPP) mode. This is believed to make APS arrays less suitable as x-ray detectors than as light detectors. In particular, because scintillators emit a much smaller number of photons than are present in a light sensing environment (such as, for example, a photography environment), the dark signal is believed to be more problematic in an x-ray detector, since the dark signal, if not corrected for, will have a greater impact on the signal-to-noise ratio. This problem is of particular significance in a bone density measuring device that uses a dual-energy source of x-ray radiation, in which there is a strong incentive to keep the energy level of the x-rays very low, since the patient will be receiving two doses.
Furthermore, it has been theorized that CMOS transistors, which are the type used in constructing APS devices, are more susceptible to damage and noise generation from high frequency radiation such as x-rays than the MOS transistors used in CCDs. Still further, it has been theorized that large APS arrays will have poor manufacturing yields.
Thus, it has heretofore never been considered to use an APS array to construct bone density measuring device. There is a need, therefore, for a new type of bone density measuring device which solves the problems of CCD-based devices by exploiting APS technology, while at the same time overcoming the real and perceived drawbacks associated with using APS arrays to detect x-rays.